Since imidazole was reported as having an inhibitory activity on thromboxane A.sub.2 synthetase [Proc. Natl. Acad. Sci., U.S.A., 74 1716 (1977) and Prostaglandins, 13, 611 (1977)], research was made extensively for developing inhibitors on thromboxane A.sub.2 synthetase and various imidazole and pyridine compounds have been disclosed in the prior art references, for example, Japanese Patent Publication (Unexamined) Nos. 54-112862, 54-112863, 54-144369, 54-163573, 55-313, 55-28927, 55-85572, 55-100368, 55-47676, 55-89266 and 56-25162; U.S. Pat. Nos. 4,226,878, 4,320,134, 4,317,828 and 4,271,170.
However, these imidazole and pyridine compounds disclosed in the prior art references are generally still not satisfactory in their effects as thromboxane A.sub.2 synthetase inhibitors, absorption from digestive tracts and toxicity.
As a result of studies for developing compounds having a strong inhibitory activity on thromboxane A.sub.2 synthetase and yet having a low toxicity, the present inventors found that the novel pyridine compounds represented by the formula (I) and their pharmaceutically acceptable salts exhibit an excellent pharmacological activity useful as an inhibitor on thromboxane A.sub.2 synthetase.